Process for improving thermostability of lubricant oils in internal combustion engines

ABSTRACT

The present invention refers to a process to improve the thermostability of lubricant oils contained in an internal combustion engine, which process comprises operating the said engine with a fuel containing one or more ashless thermostability boosters selected from aromatic amines, substituted phenols and polycyclic phenolic compounds or mixtures thereof. The present invention further refers to a fuel additive composition and to a fuel composition comprising specific nitrogen-containing dispersants, carrier oils, the said ashless thermostability boosters, and optionally corrosion inhibitors and friction modifiers.

The present invention relates to a process to improve thermostability of lubricant oils contained in an internal combustion engine for lubrication purposes, which process comprises operating the internal combustion engine with a fuel containing an effective amount of certain ashless thermostability boosters. Furthermore, the present invention relates to a novel fuel additive composition comprising selected nitrogen-containing dispersants, eventually carrier oils, the above-mentioned ashless thermostability boosters and, optionally, corrosion inhibitors and/or friction modifiers. Furthermore, the present invention relates to a gasoline fuel composition comprising a minor amount of the said fuel additive composition.

TECHNICAL BACKGROUND

The formulation of lubricant oils for an internal combustion engine, especially for lubrication of the crankcase, normally includes the incorporation of additive packages in the lubricant oils to improve or maintain the properties of the lubricant oils during service. Commonly used additives include antioxidants, corrosion inhibitors, antiwear agents extreme pressure agents, pour point depressants, dispersants, viscosity control agents, foam inhibitors and the like.

The conditions of engine operation tend to degrade the lubricant oils. Indicators of lubricant oil degradation include an increase in viscosity of the oils, sludge and gum build-up from oxidation of the oils and excessive engine wear. Especially thermal stress during engine operation is harmful to the properties of the lubricant oils. Additives described above are incorporated in the oils to help control these and other problems, in order to obtain better overall lubricant oil performance.

It has also been proposed to add antioxidant additives to the fuel with which the internal cornbustion engine is operated, in order to obtain better overall lubricant oil performance. WO 94/22988 (1) teaches to operate an internal combustion engine with a fuel containing aromatic amines or sterically hindered phenols as antioxidants, thus actually resisting degradation of the engine lubricant, engine wear and sludge build-up during engine operation and controlling viscosity of the engine oil. Improving thermostability of the lubricants is not addressed by document (1). The said fuel may also contain high molecular weight dispersants and carrier fluids to control intake valve and fuel injector cleanliness, however, without specifying such dispersants and carrier fluids.

Aromatic amines or sterically hindered phenols taught to be used to improve lubricant oil performance by document (1) are well known antioxidants for lubricants in the art, e.g. as described in EP 0 346 283 A2 (2). However, document (2) teaches to incorporate such antioxidants directly into the lubricant formulations.

The performance of antioxidant additives to improve lubricant oil performance, especially lubricant oil thermostability, in the internal combustion engine may still be unsatisfactory. It is, therefore, an object of the present invention to provide an more effective process to improve thermostability of lubricant oils contained in an internal combustion engine for lubrication purposes.

BRIEF DESCRIPTION OF THE INVENTION

A more effective process to improve the thermostability of lubricant oils contained in an internal combustion engine for lubrication purposes has now been found, which process comprises operating the internal combustion engine with a fuel containing an effective amount of one or more ashless thermostability boosters selected from

(A) aromatic amines of general formula (I)

-   -   wherein     -   R1 designates hydrogen or a C₁- to C₂₄-hydrocarbyl residue,     -   R2 designates a C₁- to C₂₄-hydrocarbyl residue which may be         substituted by one or more hydroxyl groups and/or amino groups         and/or which may be interrupted by one or more oxygen and/or         sulphur atoms,     -   R3 and R4 designate independently from each other hydrogen or         C₁- to C₂₄-hydrocarbyl residues;         (B) substituted phenols of general formula (II)

-   -   wherein     -   R5 designates a residue of formula —C_(n)H_(2n)—CO—OR8 or of         formula —C_(m)H_(2m)—S_(x)—R9 wherein n and m are independently         of each other the numbers 0, 1, 2 or 3, x is the number 1, 2, 3         or 4, R8 designates a C₁- to C₂₄-hydrocarbyl residue and R9         designates hydrogen or a C₁- to C₂₄-hydrocarbyl residue;     -   R5 and R6 designate independently from each other hydrogen or         C₁- to C₂₄-hydrocarbyl residues;         (C) polycyclic phenolic compounds having up to 20 benzene rings         per molecule which are obtainable by reacting a         tetrahydrobenzoxazine with one or more of the same or different         phenols and/or with one or more of the same or different         tetrahyd robenzoxazi nes;         or mixtures of (A), (B) and/or (C).

Aromatic amines (A) of formula (I) and substituted phenols (B) of formula (II) are known as anti-oxidant additives for lubricants from document (2).

Moreover, it has now been observed that a fuel additive composition essentially comprising:

(i) at least one nitrogen-containing dispersant (D) selected from

-   -   (D1) polyisobutenyl monoamines,     -   (D2) polyisobutenyl polyamines,     -   (D3) Mannich reaction products of substituted phenols with         aldehydes and mono- or polyamines, and     -   (D4) polyoxyalkylenes which are terminated by mono- or polyamino         groups;         (ii) in case of presence of (D1), (D2) or (D3), at least one         carrier oil which is substantially free of nitrogen, selected         from synthetic carrier oils and mineral carrier oils;         (iii) at least one thermostability booster selected from         aromatic amines (A) of general formula (I), substituted         phenols (B) of general formula (II), polycyclic phenolic         compounds (C) and mixtures of (A), (B) and/or (C), as referred         to above;         (iv) optionally, at least one corrosion inhibitor (E); and         (v) optionally, at least one friction modifier (F),         still further improves the thermostability of lubricant oils         contained in an internal combustion engine for lubrication         purposes. Therefore, the said fuel additive composition is also         a subject matter of the instant invention.

A further subject matter of the instant invention is a fuel composition comprising a major amount of a liquid fuel in gasoline boiling range and a minor amount of the above fuel additive composition.

DETAILS DESCRIPTION OF THE INVENTION AND PREFERRED EMBODIMENTS

In a preferred embodiment of the instant invention, the inventive process is applied to a sparkignited combustion engine which is operated with gasoline as the fuel.

“Effective amount” usually means that the one or more ashless thermostability booster (A), (B) and/or (C) are present in the fuel in an amount of from 1 to 3,000 ppm by weight, more preferably of from 10 to 1,500 ppm by weight, most preferably of from 25 to 500 ppm by weight. The most typical dosage rate in the fuel for the ashless thermostability booster (A) and/or (B) is from 50 to 250 ppm by weight, especially from 75 to 150 ppm by weight.

The Nitrogen-Containing Dispersant [Component (i)]

The polyisobutenes which are suitable for preparing the polyisobutenyl monoamines (D1) and polyisobutenyl polyamines (D2) used in the present invention include polyisobutenes which comprise at least about 20%, preferably at least 50% and more preferably at least 70% of the more reactive methylvinylidene isomer. Suitable polyisobutenes include those prepared using BF₃ catalysts. The preparation of such polyisobutenes in which the methylvinylidene isomer comprises a high percentage of the total composition is for example described in U.S. Pat. No. 4,152,499 and U.S. Pat. No. 4,605,808.

Examples of suitable polyisobutenes having a high alkylvinylidene content include Ultravis® 30, a polyisobutene having a number average molecular weight of about 1300 and a methylvinylidene content of about 74%, and Ultravis® 10, a 950 molecular weight polyisobutene having a methylvinylidene content of about 76%, both available from British Petroleum. Another example of a suitable polyisobutene having a number average molecular weight of about 1000 and a high methylvinyliden content is Glissopal® 1000, available from BASF SE.

In most instances, the polyisobutene precursors are not a pure single product, but rather a mixture of compounds having an average molecular weight in the above range. Usually, the range of molecular weights will be relatively narrow having a maximum near the indicated molecular weight.

The amine component of the polyisobutenyl monoamines or polyisobutenyl polyamines, respectively, may be derived from ammonia, a monoamine or a polyamine.

The monoamine or polyamine component comprises amines having from 1 to about 12 amine nitrogen atoms and from 1 to 40 carbon atoms. The carbon to nitrogen ratio may be between about 1:1 and about 10:1. Generally, the monoamine will contain from 1 to about 40 carbon atoms and the polyamine will contain from 2 to about 12 amine nitrogen atoms and from 2 to about 40 carbon atoms.

The amine component may be a pure single product or a mixture of compounds having a major quantity of the designated amine.

When the amine component is a polyamine, it will preferably be a polyalkylene polyamine, including alkylene diamine. Preferably, the alkylene group will contain from 2 to 6 carbon atoms, more preferably from 2, 3 or 4 carbon atoms. Examples of such polyamines include ethylene diamine, diethylene triamine, triethylene tetramine and tetraethyl-ene pentamine. Preferred polyamines are ethylene diamine and diethylene triamine.

Particularly preferred polyisobutenyl polyamines include polyisobutenyl ethylene diamine and polyisobutenyl amine. The polyisobutenyl group is substantially saturated.

The polyisobutenyl monoamines or polyisobutenyl polyamines employed in the fuel additive composition of the instant invention are prepared by conventional procedures known in the art, especially by reductive amination. Such polyisobutenyl monoamines or polyisobutenyl polyamines and their preparations are described in detail e.g. in EP-A 0 244 616.

The amine portion of the molecule may carry one or more substituents. Thus, the carbon and/or, in particular, the nitrogen atoms of the amine may carry substituents selected from hydrocarbyl groups of from 1 to about 10 carbon atoms, acyl groups of from 2 to about 10 carbon atoms, and monoketo, monohydroxy, mononitro, monocyano, lower alkyl and lower alkoxy derivatives thereof. “Lower” as used herein means a group containing from 1 to about 6 carbon atoms. At least one of hydrogen atoms on one of the basic nitrogen atoms of the polyamine may not be substituted so that at least one of the basic nitrogen atoms of the polyamine is a primary or secondary amino nitrogen.

A polyamine finding use within the scope of the present invention as amine component for the polyisobutenyl polyamines may be a polyalkylene polyamine, including substituted polyamines, e.g., alkyl and hydroxyalkyl-substituted polyalkylene polyamine. Among the polyalkylene polyamines, those containing 2 to 12 amino nitrogen atoms and 2 to 24 carbon atoms should be mentioned, in particular C₂- to C₃-alkylene polyamines. Preferably, the alkylene group contains from 2 to 6 carbon atoms, there being preferably from 2 to 3 carbon atoms between the nitrogen atoms. Such groups are exemplified by ethylene, 1,2-propylene, 2,2-dimethylpropylene, trimethylene, and 1,3,2-hydroxypropylene.

Examples of such polyamines comprise 1,2-ethylene diamine, diethylene triamine, di(trimethylene) triamine, 1,2-propylene diamine, 1,3-propylene diamine, dipropylene triamine, triethylene tetraamine, tripropylene tetraamine, tetraethylene pentamine, pentaethylene hexamine, hexamethylene diamine and dimethylaminopropylene diamine. Such amines encompass isomers such as branched-chain polyamines and previously-mentioned substituted polyamines, including hydroxy- and hydrocarbyl-substituted polyamines.

The amine component for the polyisobutenyl monoamines or polyisobutenyl polyamines also may be derived from heterocyclic polyamines, heterocyclic substituted amines and substituted heterocyclic compounds, wherein the heterocycle comprises one or more 5- to 6-membered rings containing oxygen and/or nitrogen. Such heterocyclic rings may be saturated or unsaturated and substituted with groups as defined above.

As examples of heterocyclic compounds there may be mentioned 2-methylpiperazine, N-(2hydroxyethyl)-piperazine, 1,2-bis-(N-piperazinyl)ethane, N,N′-bis(N-piperazinyl)-piperazine, 2-methylimidazoline, 3-aminopiperidine, 3-aminopyridine, N-(3-amino-propyl)-morpholine, N-(beta-aminoethyl)piperazine, N-(betaaminoethyl)piperidine, 3-amino-N-ethylpiperidine, N-(betaaminoethyl) morpholine, N,N′-di(beta-aminoethyl)-piperazine, N,N′-di(beta-aminoethyl)imidazolidone-2, 1,3-dimethyl-5(beta-amino-ethyl)hexahydrotriazine, N-(betaaminoethyl)-hexahydrotriazine, 5-(beta-aminoethyl)-1,3,5-dioxazine.

Alternatively, the amine component for the polyisobutenyl monoamines may be derived from a monoamine having the formula HNR¹⁰R¹¹ wherein R¹⁰ and R¹¹ are independently selected from the group consisting of hydrogen and hydrocarbyl of 1 to about 20 carbon atoms and, when taken together, R¹⁰ and R¹¹ may form one or more 5- or 6-membered rings containing up to about 20 carbon atoms. Preferably, R¹⁰ is hydrogen and R¹¹ is a hydrocarbyl group having 1 to about 10 carbon atoms. More preferably, R¹⁰ and R¹¹ are hydrogen. The hydrocarbyl groups may be straight-chain or branched and may be aliphatic, alicyclic, aromatic or combinations thereof. The hydrocarbyl groups may also contain one or more oxygen atoms.

Typical primary amines are exemplified by N-methylamine, N-ethylamine, N-n-propyl-amine, N-isopropylamine, N-n-butylamine, N-isobutylamine, N-sec.-butylamine, N-tert-butylamine, N-n-pentylamine, N-cyclopentylamine, N-n-hexylamine, N-cyclohexyl-amine, N-octylamine, N-decylamine, N-dodecylamine, N-octadecylamine, N-benzyl-amine, N-(2-phenylethyl)amine, 2-aminoethanol, 3-amino-1-proponal, 2-(2-amino-ethoxy)ethanol, N-(2-methoxyethyl)amine, N-(2-ethoxyethyl)amine, and the like. Preferred primary amines are N-methylamine, N-ethylamine and N-n-propylamine.

Typical secondary amines include N,N-dimethylamine, N,N-diethylamine, N,N-di-n-propylamine, N,N-diisopropylamine, N,N-di-n-butylamine, N,N-di-sec-butylamine, N,N-di-n-pentylamine, N,N-di-n-hexylamine, N,N-dicyclohexylamine, N,N-dioctylamine, N-ethyl-N-methylamine, N-methyl-N-n-propylamine, N-n-butyl-N-methylamine, N-methyl-N-octylamine, N-ethyl-N-isopropylamine, N-ethyl-N-octylamine, N,N-di-(2-hydroxy-ethyl)amine, N,N-di(3-hydroxypropyl)amine, N,N-di(ethoxyethyl)amine, N,N-di-(pro-poxyethyl)amine, and the like. Preferred secondary amines are N,N-dimethylamine, N,N-diethylamine and N,N-di-n-propylamine.

Cyclic secondary amines may also be employed to form the polyisobutenyl monoamines or polyisobutenyl polyamines used in the instant invention. In such cyclic compounds, R¹⁰ and R¹¹ of the formula hereinabove, when taken together, form one or more 5- or 6-membered rings containing up to about 20 carbon atoms. The ring containing the amine nitrogen atom is generally saturated, but may be fused to one or more saturated or unsaturated rings. The rings may be substituted with hydrocarbyl groups of from 1 to about 10 carbon atoms and may contain one or more oxygen atoms.

Suitable cyclic secondary amines include piperidine, 4-methylpiperidine, pyrrolidine, morpholine, 2,6-dimethylmorpholine, and the like.

The number average molecular weight of the polyisobutenyl monoamines or polyisobutenyl polyamines used in the instant invention is usually in the range of from 500 to 2500, typically about 550, about 750, about 1000 or about 1300. A preferred range for the number average molecular weight of the polyisobutenyl monoamines or polyisobutenyl polyamines is from 550 to 1000. As already stated for the polyisobutene precursors, the polyisobutenyl monoamines or polyisobutenyl polyamines are mostly not pure single products, but rather mixtures of compounds having number average molecular weights as indicated above. Usually, the range of molecular weights will be relatively narrow having a maximum near the indicated molecular weight.

In an especially preferred embodiment, dispersant component (i) is a polyisobutenyl monoamine (D1) with a number average molecular weight of from 550 to 1000. The said polyisobutenyl monoamine is preferably based on ammonia and preferably prepared via hydroformylation of polyisobutene and subsequent reductive amination with ammonia, as described in EP-A 0 244 616.

The Mannich reaction products of substituted phenols with aldehydes and mono- or polyamines (D3) are preferably reaction products of polyisobutene-substituted phenols with formaldehyde and mono- or polyamines such as ethylenediamine, diethylenetriamine, triethylenetetramine, tetraethylenepentamine or dimethylaminopropylamine. The polyisobutenyl-substituted phenols may stem from conventional or highly reactive polyisobutene having a number average molecular weight of from 300 to 5000. Such “polyisobutene-Mannich bases” are described in particular in EP-A 831 141.

The polyoxyalkylenes which are terminated by mono- or polyamino groups (D4), which are also called “polyetheramines”, are obtainable by reaction of C₂- to C₆₀-alkanols, C₆- to C₃₀-alkanediols, mono- or di-C₂- to C₃₀-alkylamines, C₁- to C₃₀-alkylcyclohexanols or C₁- to C₃₀-alkylphenols with from 1 to 30 mol of ethylene oxide and/or propylene oxide and/or butylene oxide per hydroxyl group or amino group and by subsequent reductive amination with ammonia, monoamines or polyamines. Such products are described in particular in EP-A 310 875, EP-A 356 725, EP-A 700 985 and U.S. Pat. No. 4 877 416. In most cases, such products also have carrier oil properties. Typical examples of these are the corresponding reaction products of tridecanol butoxylates, isotridecanol butoxylates, isononylphenol butoxylates and polyisobutenol butoxylates and propoxylates with ammonia.

The Carrier Oil [Component (ii)]

The fuel-soluble, nonvolatile carrier oil of component (ii) is to be used as a necessary part of the fuel additive composition of the instant invention only in combination with components (D1), (D2) and (D3). The carrier oil is a chemically inert hydrocarbon-soluble liquid vehicle. The carrier oil of component (ii) may be a synthetic oil or a mineral oil; for the instant invention, a refined petroleum oil is also understood to be a mineral oil. Such carrier oils (also called carrier fluids) are believed to act as a carrier for the fuel additives and to assist in removing and retarding deposits.

The carrier oil of component (ii) is typically employed in amounts ranging from about 10 to about 2500 ppm by weight of the gasoline fuel, preferably from 30 to 1000 ppm of the gasoline fuel. Preferably, the ratio of carrier oil (ii) to nitrogen-containing dispersant (D1), (D2) or (D3) in the fuel additive composition as well as in the gasoline fuel will range from 0.25:1 to 10:1, typically from 0.4:1 to 4:1.

Examples for suitable mineral carrier oils are in particular those of viscosity class Solvent Neutral (SN) 500 to 2000, as well as aromatic and paraffinic hydrocarbons and alkoxyalkanols. Another useful mineral carrier oil is a fraction known as “hydrocrack oil” which is obtained from refined mineral oil (boiling point of approximately 360 to 500° C.; obtainable from natural mineral oil which is isomerized, freed of paraffin components and catalytically hydrogenated under high pressure).

Examples for synthetic carrier oils which can be used for the instant invention are olefin polymers with a number average molecular weight of from 400 to 1800, based on poly-alpha-olefins or poly-internal-olefins, especially those based on polybutene or on polyisobutene (hydrogenated or nonhydrogenated). Further examples for suitable synthetic carrier oils are polyesters, polyalkoxylates, polyethers, alkylphenol-initiated polyethers, and carboxylic acids of long-chain alkanols.

Examples for suitable polyethers which can be used for the instant invention are compounds containing polyoxy-C₂-C₄-alkylene groups, especially polyoxy-C₃-C₄-alkylene groups, which can be obtained by reacting C₁-C₃₀-alkanols, C₂-C₆₀-alkandiols, C₁-C₃₀-alkylcyclohexanols or C₁-C₃₀alkylphenols with 1 to 30 mol ethylene oxide and/or propylene oxide and/or butylene oxides per hydroxyl group, especially with 1 to 30 mol propylene oxide and/or butylene oxides per hydroxyl group. This type of compounds is described, for example, in EP-A 310 875, EP-A 356 725, EP-A 700 985 and U.S. Pat. No. 4,877,416.

Typical examples for suitable polyethers are tridecanol butoxylates, isotridecanol butoxylates, isononylphenol butoxylates, polyisobutenol butoxylates and polyisobutenol propoxylates.

Hydrocarbyl-terminated poly(oxyalkylene) polymers which may be employed in the present invention as component (ii), are monohydroxy compounds, i.e., alcohols, and are often termed monohydroxy polyethers, or polyalkylene glycol monohydrocarbylethers, or “capped” poly(oxyalkylene).

The hydrocarbyl-terminated poly(oxyalkylene) alcohols may be produced by the addition of lower alkylene oxides, such as ethylene oxide, propylene oxide, the butylene oxides, or the pentylene oxides to the hydroxy compound under polymerization conditions. Methods of production and properties of these polymers are disclosed in U.S. Pat. Nos. 2,841,479 and 2,782,240 and Kirk-Othmer's “Encyclopedia of Chemical Technology”, 2nd Ed. Volume 19, p. 507. In the polymerization reaction, a single type of alkylene oxide may be employed, e.g., propylene oxide, in which case the product is a homopolymer, e.g., a poly(oxyalkylene) propanol. However, copolymers are equally satisfactory and random copolymers are readily prepared by contacting the hydroxyl-containing compound with a mixture of alkylene oxides, such as a mixture of propylene and butylene oxides. Block copolymers of oxyalkylene units also provide satisfactory poly(oxyalkylene) polymers for the practice of the present invention. Random polymers are more easily prepared when the reactivities of the oxides are relatively equal. In certain cases, when ethylene oxide is copolymerized with other oxides, the higher reaction rate of ethylene oxide makes the preparation of random copolymers difficult. In either case, block copolymers can be prepared. Block copolymers are prepared by contacting the hydroxyl-containing compound with first one alkylene oxide, then the others in any order, or repetitively, under polymerization conditions. A particular block copolymer is represented by a polymer prepared by polymerizing propylene oxide on a suitable monohydroxy compound to form a poly(oxypropylene) alcohol and then polymerizing butylene oxide on the poly(oxyalkylene) alcohol.

In general, the poly(oxyalkylene) polymers are mixtures of compounds that differ in polymer chain length. However, their properties closely approximate those of the polymer represented by the average composition and molecular weight.

Examples of carboxylic esters of long-chain alkanols are esters of mono-, di- and tricarboxylic acids with long-chain alkanols or polyhydric alcohols such as described e.g. in DE-A 38 38 918. Suitable mono-, di- and tricarboxylic acids are aliphatic or aromatic carboxylic acids. Suitable alkanols and polyhydric alcohols contain 6 to 24 carbon atoms. Typical examples of such esters are the adipates, phthalates, iso-phthalates, terephthalates and trimellitates of isooctanol, isononanol, isodecanol and isotridecanol, e.g. di-n-tridecyl phthalate or di-iso-tridecyl phthalate.

Examples for particularly useful synthetic carrier oils are alcohol-initiated polyethers containing about 5 to 35, e.g. 5 to 30 C₃-C₆-alkylenoxide units, such as propylene oxide, n-butylene oxide and iso-butylene oxide units or mixtures thereof. Non-limiting examples for alcoholic starters are long-chain alkanols or phenols substituted by long-chain alkyl groups, where the alkyl group preferably is linear or branched C₆- to C₁₈-alkyl. Preferred examples for the alcoholic starters are tridecanol and nonylphenol.

Further suitable synthetic carrier oils are alkoxylated alkylphenols, such as described e.g. in DE-A 10 102 913.

Preferably, synthetic carrier oils are used. Preferred synthetic carrier oils are alkanol alkoxylates, in particular alkanol propoxylates and alkanol butoxylates.

In an especially preferred embodiment, carrier oil component (ii) comprises at least one polyether obtained from C₁- to C₃₀-alkanols, especially C₆- to C₁₈-alkanols, or C₂- to C₆₀-alkandiols, especially C₈- to C₂₄-alkandiols, and from 1 to 30 mol, especially 5 to 30 mol, in sum, of propylene oxide and/or butylene oxides. Other synthetic carrier oils and/or mineral carrier oils may be present in component (B) in minor amounts.

The Thermostability Booster [Component (iii)]

“Hydrocarbyl residue” for R1 to R4 in general formula (I) for booster (A) and for R6 to R9 in general formula (II) for booster (B) shall mean a residue which is essentially composed of carbon and hydrogen, however, it can contain in small amounts hetero-atomes, especially oxygen and/or nitrogen, and/or functional groups, e.g. hydroxyl groups and/or carboxylic groups, to an extent which does not distort the predominantly hydrocarbon character of the residue. Hydrocarbyl residues are preferably alkyl, alkenyl, alkinyl, cycloalkyl, aryl, alkylaryl or arylalkyl groups. Especially preferred hydrocarbyl residues for R1 to R4 and R6 to R9 are linear or branched C₁-to C₂₄-alkyl groups, more preferably linear or branched C₁- to C₁₄-alkyl groups, most preferably linear or branched C₁- to C₈-alkyl groups, or C₂- to C₂₄-alkenyl groups, more preferably linear or branched C₂- to C₁₄-alkenyl groups, most preferably linear or branched C₂- to C₈-alkenyl groups.

Examples for suitable linear or branched C₁- to C₂₄-alkyl residues for R1 to R4 and R6 to R9 are: methyl, ethyl, n-propyl, iso-propyl, n-butyl, iso-butyl, sec.-butyl, tert-butyl, n-pentyl, tert-pentyl, 2-methylbutyl, 3-methylbutyl,1,1-dimethylpropyl,1,2-dimethylpropyl, n-hexyl, 2-methylpentyl, 3-methylpentyl, 4-methylpentyl, 1,1-dimethylbutyl, 1,2-di-methylbutyl, 1,3-dimethylbutyl, 2-ethylbutyl, n-heptyl, 1-methylhexyl, 2-methylhexyl, 3-methylhexyl, 4-methylhexyl, 5-methylhexyl, 1,1-dimethylpentyl, 1,2-dimethylpentyl, 2,2-dimethylpentyl, 2,3-dimethylpentyl, 2,4-dimethylpentyl, 2,5-dimethylpentyl, 2-diethyl-pentyl, 3-diethylpentyl, n-octyl, iso-octyl, tert-octyl, 1-methylheptyl, 2-methylheptyl, 3-methylheptyl, 4-methylheptyl, 5-methylheptyl, 6-methylheptyl, 1,1-dimethylhexyl, 1,2-dimethylhexyl, 2,2-dimethylhexyl, 2,3-dimethylhexyl, 2,4-dimethylhexyl, 2,5-dimethyl-hexyl, 2,6-dimethylhexyl, 2-ethylhexyl, 3-ethylhexyl, 4-ethylhexyl, n-nonyl, iso-nonyl, n-decyl, 1-propylheptyl, 2-propylheptyl, 3-propylheptyl, n-undecyl, n-dodecyl, n-tridecyl, iso-tridecyl, tetradecyl, pentadecyl, hexadecyl, heptadecyl, octadecyl, nonadecyl, eicosyl, heneicosyl, docosanyl, tricosanyl and tetracosanyl.

Examples for suitable linear or branched C₂- to C₂₄-alkenyl and -alkinyl residues for R1 to R4 and R6 to R9 are: vinyl, allyl, methallyl, oleyl and propin-2-yl.

Examples for suitable C₃- to C₂₄-cycloalkyl residues for R1 to R4 and R6 to R9 are: cyclopropyl, cyclobutyl, 2-methylcyclohexyl, 3-methylcyclohexyl, 4-methylcyclohexyl, 2,3-dimethyl-cyclohexyl, 2,4-dimethylcyclohexyl, 2,5-dimethylcyclohexyl, 2,6-dimethyl-cyclohexyl, 3,4-dimethylcyclohexyl, 3,5-dimethylcyclohexyl, 2-ethylcyclohexyl, 3-ethyl-cyclohexyl, 4-ethylcyclohexyl, cyclooctyl and cyclodecyl.

Examples for suitable C₇- to C₂₄-aryl, -alkylaryl or -arylalkyl residues for R1 to R4 and R6 to R9 are: naphthyl, tolyl, xylyl, n-octylphenyl, n-nonylphenyl, n-decylphenyl, benzyl, 1-phenyl-ethyl, 2-phenylethyl, 3-phenylpropyl and 4-butylphenyl.

Moreover, hydrocarbyl residue R2 may be substituted by one or more, especially by one or two hydroxyl groups, and/or by one or more, especially by one or two amino groups. Such amino groups may by unsubstituted amino (—NH₂) or mono-C₁- to C₂₄-hydrocarbyl-substituted or di-C₁-to C₂₄-hydrocarbyl-substituted amino. Typical examples for such residues are 2-hydroxyethyl, 3-hydroxypropyl, o-, m- or p-hydroxyphenyl, o-, m- or p-hydroxybenzyl, dihydroxyphenyl, 2-aminoethyl, 2-(N-methylamino)-ethyl, 2-(N,N-dimethylamino)-ethyl, o-, m- or p-aminophenyl, o-, m- or p-(N-methylamino)-phenyl, o-, m- or p-(N,N-dimethylamino)-phenyl, p-(N-phenylam ino)-phenyl, p-[N-(C₁- to C₂₄-alkylphenyl)amino]-phenyl and p-[N-(C₁- to C₂₄-alkoxyphenyl)amino]-phenyl.

Moreover, hydrocarbyl residue R2 may be interrupted by one or more, especially by one, two, three, four or five sulphur or especially oxygen atoms. In addition, such ether or thioether structures may carry one or more hydroxyl groups. Typical examples for such residues are —C₂H₄-(O—C₂H₄)_(p)—OH, —C₃H₆—(O—C₃H₆)_(q)—OH, —C₄H₈—(O—C₄H₈)_(r)—OH, o-, m- or p-phenylen-(O—C₂H₄)_(p)—OH, o-, m- or p-phenylen-(O—C₃H₆)_(q)—OH and o-, m- or p-phenylen-(O—C₄H₈)_(r)—OH, p, q and r each being a number of from 0 to 5.

Moreover, hydrocarbyl residue R8 in formula —C_(n)H_(2n)—CO—OR8 for R5 may designate a hydrocarbyl residue containing a carboxylic ester function, a terminal hydroxyphenyl group and may optionally be interrupted by one or more sulphur atoms, thus doubling the functional chemical structure of the substituted phenols (B). Typical examples for such doubled substituted phenols are Ar—C_(n)H_(2n)—CO—O—CH₂CH₂—O—C_(n)H_(2n)—Ar and Ar—C_(n)H_(2n)—CO—O—CH₂CH₂—S—CH₂CH₂—O—CO—C_(n)H_(2n)—Ar, Ar being the residue of general formula (II) minus R5.

Moreover, hydrocarbyl residue R9 in formula —C_(m)H_(2m)—S_(x)—R9 for R5 may designate a hydrocarbyl residue containing a carboxylic ester function and optionally a terminal hydroxyphenyl group, thus partly also doubling the functional chemical structure of the substituted phenols (B). Typical examples for such substituted phenols containing sulphur atoms are Ar—C_(m)H_(2n)—S—C_(m)H_(2m)-C₁-C₂₄-alkyl and Ar—S—Ar, Ar being the residue of general formula (II) minus R5.

In a preferred embodiment of the instant invention, in general formula (I) for the aromatic amines (A) R1 designates hydrogen, R2 designates a phenyl residue which may be substituted by one or two C₁- to C₂₄-alkyl residues and R3 and R4 designate independently from each other hydrogen or C₁- to C₂₄-alkyl residues. More preferably, R1 and R3 each designate hydrogen, R2 designates a phenyl residue which is substituted by one or two branched C₃- to C₁₀-alkyl residues, especially by one or two C₄- to C₈-alkyl residues with tertiary carbon atoms, and R4 designates a branched C₃- to C₁₀-alkyl residue, especially a C₄- to C₈-alkyl residue with a tertiary carbon atom. Typical examples for such aromatic amines (A) are di-(C₄- to C₈-alkylphenyl)-amines with the same C₄- to C₈-alkyl residues, which exhibit a tertiary carbon atom, e.g. tert-butyl or tert-octyl (sold under the tradename of Irganox® L 57 by BASF SE).

In another preferred embodiment of the instant invention, in general formula (II) for the substituted phenols (B) R5 designates a residue of formula —C_(n)H_(2n)—CO—OR8 wherein n is the number 0, 1, 2 or 3, R8 designates a C₁- to C₂₄-alkyl residue and R5 and R6 designate independently from each other C₁- to C₂₄-hydrocarbyl residues. More preferably, R5 designates a residue of formula —C_(n)H_(2n)—CO—OR8 wherein n is the number 3 or especially 2, R8 designates a linear or especially branched C₄- to C₁₂-alkyl residue and R5 and R6 designate different or the same C₄-to C₈-alkyl residues, which exhibit a tertiary carbon atom. A typical example for such substituted phenols (B) is 3-[3,5-di-tert-butyl-4-hydroxyphenyl]-propionic acid esterified with branched C₄- to C₁₂-alkanols like iso-octanol (sold under the tradename of Irganox® L 135 by BASF SE).

The polycyclic phenolic compounds having up to 20 benzene rings per molecule which are obtainable by reacting a tetrahydrobenzoxazine with one or more of the same or different phenols and/or with one or more of the same or different tetrahydrobenzoxazines (C) are described in WO 2007/099048 A2 as antioxidants for inanimate organic material in general and especially for jet fuels.

According to WO 2007/099048 A2, the compounds (C) are polycyclic phenolic compounds which have up to 20 benzene rings per molecule and are obtainable by reacting a tetrahydrobenzoxazine of the general formula (III)

in which the substituent R11 is a hydrocarbyl radical which has from 1 to 3000 carbon atoms and may be interrupted by one or more heteroatoms from the group of O and S and/or by one or more NR16 moieties, where R16 is a hydrogen atom or a C₁- to C₄-alkyl radical, and in which the substituents R12, R13, R14 and R15 are each independently hydrogen atoms, hydroxyl groups or hydrocarbyl radicals which have in each case from 1 to 3000 carbon atoms and may be interrupted by one or more heteroatoms from the group of O and S and/or by one or more NR16 moieties, where R16 is as defined above, with one or more of the same or different phenols of the general formula (IV)

in which the substituents R17, R18, R19 and R20⁰ are each independently hydrogen atoms, hydroxyl groups or hydrocarbyl radicals which have in each case from 1 to 3000 carbon atoms and may be interrupted by one or more heteroatoms from the group of O and S and/or by one or more NR16 moieties, where R16 is as defined above, and/or with one or more of the same or different tetrahydrobenzoxazines of the general formula (III), where the substituent R14 may also be a radical of the formula Z and the substituent R19 may also be a radical of the formula Z′

in which the substituents R11, R12, R13, R15, R17, R18 and R20 are each as defined above, the substituent R17 may also be a radical derived from a tetrahydrobenzoxazine of the general formula (III), the substituent R25 is hydrogen or a radical derived from a tetrahydrobenzoxazine of the general formula (III), and the substituents R21 and R22 may be the same or different and are each hydrogen or a C₁- to C₁₀-alkyl radical, and in which the substituents R12 and R13 or R13 and R14 or R14 and R15, together with the substructure —O—CH₂—NR23-CH₂— attached to the benzene ring, may also form a second tetrahydrooxazine ring, or the substituents R12 and R13 and R14 and R15, together with the substructures —O—CH₂—NR23-CH₂— and —O—CH₂—NR24-CH₂— attached to the benzene ring, may also form a second and a third tetrahydrooxazine ring, where R23 and R24 are each independently hydrocarbyl radicals which have in each case from 1 to 3000 carbon atoms and may be interrupted by one or more heteroatoms from the group of O and S and/or by one or more NR16 moieties, where R16 is as defined above, with the proviso that at least one of the substituents R11, R12, R13, R14, R15, R17, R18, R19, R20, R23 or R24 has from 13 to 3000 carbon atoms, and the remaining substituents from the group of R11, R12, R13, R14, R15, R17, R18, R19, R20, R23 or R24, when they are hydrocarbyl radicals, each have from 1 to 20 carbon atoms.

For further structural details, preferred embodiments and examples of these polycyclic phenolic compounds, reference is made to WO 2007/099048 A2.

The ashless thermostability boosters (A), (B) and (C) of the instant invention can be present in the fuel composition and in the fuel additive composition as single species or as mixtures of two, three or more species. For example, mixtures of two or more species of (A), mixtures of two or more species of (B), mixtures of two or more species of (C), mixtures of one or more species of (A) with one or more species of (B), mixtures of one or more species of (A) with one or more species of (C), mixtures of one or more species of (B) with one or more species of (C) or mixtures of one or more species of (A) with one or more species of (B) and with one or more species of (C) can be used.

The Corrosion Inhibitor [Component (iv)]

Suitable corrosion inhibitor components (E), if any, are usually selected from aliphatic dibasic acids and their salts (including dimeric monocarboxylic acids and alkyl-substi-tuted alkenyl-substituted succinic acid amides and imides and their salts), ammonium salts of organic carboxylic acids and heterocyclic aromatics (for nonferrous metal corrosion protection). Such products are commercially available.

The Friction Modifier [Component (v)]

Suitable friction modifier components (F), which are sometimes also called lubricity additives, if any, are usually selected from fatty acids, alkenylsuccinic esters, bis-(hydroxyalkyl)fatty amines, hydroxylacetamides and castor oil. Such products are commerically available. Special suitable friction modifiers are also the reaction products from carboxylic acids and alkanolamines, as described in WO 2009/050256, and nitrogen-containing heterocycles such as tolutriazoles, as decribed in WO 2006/015800.

The Fuel Additive Composition

The present fuel additive composition of components (i), (ii), (iii) and optionally (iv) and (v) normally comprises from 5 to 75% by weight, preferably from 25 to 50% by weight of (i), from 5 to 75% by weight, preferably from 25 to 50% by weight of (ii), from 1 to 75% by weight, preferably from 2 to 50% by weight of (iii), from 0 to 40% by weight, preferably from 0.5 to 40% by weight of (iv) and from 0 to 40% by weight, preferably from 0.5 to 40% by weight of (v). In each case, the sum of the amounts of all five components (i), (ii), (iii), (iv) and (v) results in 100% by weight.

The present fuel additive composition may be formulated as a concentrate, using an inert stable oleophilic (i.e., dissolves in fuel) organic solvent boiling in the range of about 65° C. to 300° C. Preferably, an aliphatic or an aromatic hydrocarbon solvent is used, such as benzene, toluene, xylene or higher-boiling aromatics or aromatic thinners. Aliphatic alcohols of about 3 to 8 carbon atoms, such as isopropanol, isobutylcarbinol, n-butanol, 2-ethylhexanol, and the like, in combination with hydrocarbon solvents, are also suitable for use in such concentrate. In the concentrate, the amount of the present fuel additive composition of components (i), (ii), (iii) and optionally (iv) and (v) will be ordinarily at least 10% by weight to about 90% by weight, as for example 40 to 85 weight percent or 50 to 80 weight percent.

In gasoline fuels, other fuel additives may be employed with the additives of the present invention, including, for example, oxygenates, such as tert-butyl methyl ether, antiknock agents, such as methylcyclopentadienyl manganese tricarbonyl, and other dispersants/detergents, such as various hydrocarbyl amines, or succinimides. A list of suitable other detergent additives is for example given in WO 00/47698 or in EP-A 1 155 102.

Also included may be lead scavengers, such as aryl halides, e.g., dichlorobenzene, or alkyl halides, e.g., ethylene dibromide. In addition, antioxidants, metal deactivators, pour point depressants, corrosion inhibitors and demulsifiers may be present.

An interaction between components (i), (iii), (iii) and optionally (iv) and (v) is necessary to achieve the desired improvement of thermostability of lubricant oils in the internal combustion engine. Preferably, the weight ratio of dispersant component (i) to ashless thermostability booster component (iii) in the inventive fuel additive composition is in the range of from 0.25:1 to 15:1, especially of from 1:1 to 5:1, thus providing the best improvement of thermostability of lubricant oils in the internal combustion engine.

The Fuel Composition

The fuel additive composition of the present invention will preferably be employed in a liquid hydrocarbon distillate fuel boiling in the gasoline range. It is in principle suitable for use in all types of gasoline, including “light” and “severe” gasoline species. The gasoline fuels may also contain amounts of other fuels such as, for example, ethanol.

The proper concentration of the instant fuel additive composition necessary in order to achieve the desired improvement of thermostability of lubricant oils in the internal combustion engine varies depending upon the type of fuel employed, and may also be influenced by the presence of other detergents, dispersants and other additives, etc. Generally, however, from 150 to 10.000 weight ppm by weight, especially from 250 to 2800 weight ppm by weight, of the instant fuel additive composition of components (i), (ii), (iii) and optionally (iv) and (v) per part of base fuel is needed to achieve the best results.

In a preferred embodiment of the present invention, dispersant component (i) is present in the present fuel composition at a level of from 50 to 3000 ppm, preferably from 75 to 1000 ppm, more preferably from 100 to 750 ppm, most preferably from 125 to 500 ppm, carrier oil component (ii), if present, at a level of from 10 to 2500 ppm, preferably from 30 to 1000 ppm, more preferably from 50 to 700 ppm, most preferably from 60 to 400 ppm, ashless thermostability booster component (iii) at a level of from 1 to 3000 ppm, preferably from 5 to 1000 ppm, more preferably from 10 to 500 ppm, most preferably from 25 to 300 ppm, corrosion inhibitor component (iv), if present, at a level of from 2 to 100 ppm, preferably from 4 to 20 ppm, and friction modifier component (v), if present, at a level of from 5 to 2000 ppm, preferably from 10 to 800 ppm, more preferably from 25 to 500 ppm, most preferably from 40 to 300 ppm. All ppm values above refer to the weight.

Typically, gasoline fuels, which may be used according to the present invention exhibit, in addition, one or more of the following features:

The aromatics content of the gasoline is preferably not more than 50 volume % and more preferably not more than 35 volume %.

The sulfur content of the gasoline is preferably not more than 100 ppm by weight and more preferably not more than 50 ppm by weight.

The gasoline has an olefin content of not more than 21 volume %, preferably not more than 18 volume %, and more preferably not more than 10 volume %.

The gasoline has a benzene content of not more than 1.0 volume % and preferably not more than 0.9 volume %.

The gasoline has an oxygen content of not more than 45 weight %, preferably from 0 to 45 weight %, and most preferably from 0.1 to 3.7 weight % (first type) or most preferably from 2.7 to 45 weight % (second type). The gasoline of the second type mentioned above is a mixture of lower alcohols such as methanol or especially ethanol, which derive preferably from natural source like plants, with mineral oil based gasoline, i.e. usual gasoline produced from crude oil. An example for such gasoline is “E 85”, a mixture of 85 volume % of ethanol with 15 volume % of mineral oil based gasoline. Also a fuel containing 100% of a lower alcohol, especially ethanol, is suitable.

The content of alcohols, especially lower alcohols, and ethers in a gasoline of the first type mentioned in the above paragraph is normally relatively low. Typical maximum contents are for methanol 3 volume %, for ethanol 10 volume %, for isopropanol 10 volume %, for tert-butanol 7 volume %, for iso-butanol 10 volume %, and for ethers containing 5 or more carbon atoms in the molecule 15 volume %.

For example, a gasoline which has an aromatics content of not more than 38 volume % and at the same time an olefin content of not more than 18 volume %, a sulfur content of not more than 50 ppm by weight, a benzene content of not more than 1.0 volume % and an oxygen content of from 0.1 to 3.7 weight % may be applied.

The summer vapor pressure of the gasoline is usually not more than 90 kPa and preferably not more than 60 kPa (at 37° C.).

The research octane number (“RON”) of the gasoline is usually from 90 to 100. A usual range for the corresponding motor octane number (“MON”) is from 80 to 90.

The above characteristics are determined by conventional methods (DIN EN 228).

EXPERIMENTAL PART

The following examples are presented to illustrate specific embodiments of this invention and are not to be construed in any way as limiting the scope of the invention.

Example 1 Determination of Thermostability of Lubricant Oil in a Mercedes-Benz M102 E Spark-Ignited Combustion Engine

According to the test cycle procedure of CEC F-05-93, a Mercedes-Benz M102 E spark-ignited combustion engine was run for each 200 hour periods operated by a commercially available E 10 gasoline (DIN 51626-1) containing:

-   -   Run I: no additives;     -   Run II: dispersant (115 ppm by weight of a commercially         available polyisobutenyl monoamine) and carrier oil (140 ppm by         weight of a commercially available polyether carrier oil);     -   Run III: dispersant (115 ppm by weight of a commercially         available polyisobutenyl monoamine), carrier oil (140 ppm by         weight of a commercially available polyether carrier oil) and         ashless thermostability booster [100 ppm by weight of         di-(tert-butyl-phenyl)-amine].

The lubricant oil used in the crankcase of the engine was CEC Reference Oil RL-223/5. Thermostability of the lubricant oil was determined as the onset temperature [in ° C.] obtained by high pressure differential scanning calorimetry (HPDSC) using an oxygen atmosphere and applying a heating rate of 5 K/min (for Run III see diagram below).

Example 2 Determination of Thermostability of Lubricant Oil in a Mercedes-Benz M111 Spark-Ignited Combustion Engine

According to the test cycle procedure of CEC F-20-98, a Mercedes-Benz M111 spark-ignited combustion engine was run for each 200 hour periods operated by a commercially available E 10 gasoline (DIN 51626-1) containing:

-   -   Run I: no additives;     -   Run II: dispersant (115 ppm by weight of a commercially         available polyisobutenyl monoamine) and carrier oil (140 ppm by         weight of a commercially available polyether carrier oil);     -   Run III: dispersant (115 ppm by weight of a commercially         available polyisobutenyl monoamine), carrier oil (140 ppm by         weight of a commercially available polyether carrier oil) and         ashless thermostability booster [100 ppm by weight of         di-(tert-butyl-phenyl)-amine].

The lubricant oil used in the crankcase of the engine was CEC Reference Oil RL-223/5. Thermostability of the lubricant oil was determined as the onset temperature [in ° C.] obtained by high pressure differential scanning calorimetry (HPDSC) using an oxygen atmosphere and applying a heating rate of 5 K/min.

Example 3 Determination of Thermostability of Lubricant Oil in a Chevrolet 350 V8 Spark-Ignited Combustion Engine

According to the test cycle procedure of simulation of sequence IIIG (ASTM D 7320), a Chevrolet 350 V8 spark-ignited combustion engine was run for each 100 hour periods operated by a commercially available gasoline (Commercial US RUL E 10) containing:

-   -   Run II: dispersant (115 ppm by weight of a commercially         available polyisobutenyl monoamine) and carrier oil (140 ppm by         weight of a commercially available polyether carrier oil);     -   Run III: dispersant (115 ppm by weight of a commercially         available polyisobutenyl monoamine), carrier oil (140 ppm by         weight of a commercially available polyether carrier oil) and         ashless thermostability booster [100 ppm by weight of         di-(tert-butyl-phenyl)-amine];     -   Run IV: dispersant (115 ppm by weight of a commercially         available polyisobutenyl monoamine), carrier oil (140 ppm by         weight of a commercially available polyether carrier oil) and         ashless thermostability booster [100 ppm by weight of         3-[3,5-di-tert-butyl-4-hydroxyphenyl]-propionic acid esterified         with iso-octanol].

The lubricant oil used in the crankcase of the engine was Castrol GTX 5W-30. Thermostability of the lubricant oil was determined as the onset temperature [in ° C.] obtained by high pressure differential scanning calorimetry (HPDSC) using an oxygen atmosphere and applying a heating rate of 5 K/min.

The table below shows the results of the thermostability determinations [in ° C]:

Example 1 Example 2 Example 3 Fresh lubricant oil 235 235 246 Run I (aged lubricant oil) 215 213 — Run II 212 211 177 Run III 221 222 184 Run IV — — 192

The diagram below shows exemplarily the determination for Run Ill of Example 1. 

1. A process for improving thermostability of a lubricant oil in an internal combustion the process, comprising: operating an internal combustion engine with a fuel comprising an effective amount of one or more ashless thermostability boosters selected from the group consisting of: (A) an aromatic amine of formula (I):

(B) a substituted phenol of formula (II):

and (C) a polycyclic phenolic compound having up to 20 benzene rings per molecule, wherein: R1 designates hydrogen or a C₁- to C₂₄-hydrocarbyl residue; R2 designates a C₁- to C₂₄-hydrocarbyl residue which is optionally a) substituted by one or more hydroxyl groups, one or more amino groups, or both, b) interrupted by one or more oxygen, one or more sulphur, or both, or both a) and b); R3 and R4 designate independently hydrogen or a C₁- to C₂₄-hydrocarbyl residue; R5 designates a residue of formula —C_(n)H_(2n)—CO—OR8 or of a residue of formula —C_(m)H_(2m)—S_(x)—R9 wherein n and m are independently 0, 1, 2 or 3; x is the number 1, 2, 3 or 4; R8 designates a C₁- to C₂₄-hydrocarbyl residue; and R9 designates hydrogen or a C₁- to C₂₄-hydrocarbyl residue; R5 and R6 designate independently hydrogen or a C₁- to C₂₄-hydrocarbyl residues residue; the polycyclic phenolic compound is obtainable by a process comprising reacting a tetrahydrobenzoxazine with one or more of the same or different phenols, one or more of the same or different tetrahydrobenzoxazines, or both.
 2. The process according to claim 1, wherein the fuel is gasoline and the internal combustion engine is a spark-ignited combustion engine.
 3. The process according to claim 1, wherein the effective amount of the one or more ashless thermostability boosters in the fuel is from 1 to 3,000 ppm by weight.
 4. The process according to claim 1, wherein R1 designates hydrogen; R2 designates a phenyl residue which is optionally substituted by one or two C₁- to C₂₄-alkyl residues; and R3 and R4 designate independently hydrogen or a C₁- to C₂₄-alkyl residue.
 5. The process according to claim 1, wherein R5 designates a residue of formula —C_(n)H_(2n)—CO—OR8, wherein n is 0, 1, 2 or 3; R8 designates a C₁- to C₂₄-alkyl residue and R5 and R6 designate independently a C₁- to C₂₄-hydrocarbyl residue.
 6. A fuel additive composition, comprising: (i) at least one nitrogen-containing dispersant (D) selected from the group consisting of: (D1) a polyisobutenyl monoamine, (D2) a polyisobutenyl polyamine, (D3) a Mannich reaction product of a substituted phenol with an aldehyde and a mono- or polyamine, and (D4) a polyoxyalkylene which is terminated by a mono- or polyamino group; (ii) in case of presence of (D1), (D2) or (D3), at least one carrier oil, which is substantially free of nitrogen, selected from the group consisting of a synthetic carrier oil and a mineral carrier oil; (iii) the one or more ashless thermostability boosters of claim 1; (iv) optionally, a corrosion inhibitor (E); and (v) optionally, a friction modifier (F).
 7. The fuel additive composition according to claim 6, wherein the at least one nitrogen-containing dispersant (i) is a polyisobutenyl monoamine (D1) with a number average molecular weight of from 550 to
 1000. 8. The fuel additive composition according to claim 6, wherein the weight ratio of the at least one nitrogen-containing dispersant (i) to the one or more ashless thermostability boosters (iii) is from 0.25:1 to 15:1.
 9. A fuel composition, comprising: a major amount of a liquid fuel in gasoline boiling range and a minor amount of the fuel additive composition according to claim
 6. 10. The fuel composition according to claim 9, wherein the fuel additive composition comprises: the at least one nitrogen-containing dispersant (i) in an amount of from 50 to 3000 ppm, the at least one carrier oil (ii), if present, in an amount of from 10 to 2500 ppm, the one or more ashless thermostability boosters (iii) in an amount of from 1 to 3000 ppm, the corrosion inhibitor (iv), if present, in an amount of from 2 to 100 ppm, and the friction modifier (v), if present, in an amount of from 5 to 2000 ppm. 